ovl: remove upper umask handling from ovl_create_upper()

This is already done by vfs_prepare_mode() when creating the upper object
by vfs_create(), vfs_mkdir() and vfs_mknod().

No regressions have been observed in xfstests run with posix acls turned
off for the upper filesystem.

Fixes: 1639a49c ("fs: move S_ISGID stripping into the vfs_*() helpers")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>